This invention relates to optical fibre apparatus. In particular, but not exclusively, the invention relates to optical fibres for delivering a high power laser beam to a workpiece. The invention also relates to a configuration of either the input or output end of an optical fibre assembly that makes the assembly much less liable to fail due to either input beam/fibre misalignment or, at the output end, due to back reflection of laser radiation or backward coupling of broadband optical radiation generated at a workpiece by laser beam interaction.
It is generally preferred to use an optical fibre delivery system for delivering a laser beam to a workpiece for material processing operations. The lasers used may typically be Nd:YAG lasers operating at wavelengths such as 1060 nm. The advantages of using optical fibre delivery systems are well known and include the ability to scan a laser beam easily over a workpiece, the ability to site the laser remotely from the workpiece and the ability to distribute power from a single laser to a plurality of remote work stations.
The structure of optical fibres and the principles of operation are well known. An optical fibre for transmitting a laser beam generally comprises a central core of circular cross section surrounded by a cladding layer. Usually a buffer layer and an outer jacket layer are also provided to protect and strengthen the core and cladding layers.
Ideally, a laser beam is introduced into an optical fibre by being directed onto the core region at an input face of the fibre. In practice, however, often a portion of the laser beam may unintentionally also impinge upon the cladding region of the input face of the fibre, and this can often result in significant laser power entering the cladding layer. This can arise because the width of the laser beam at the input face of the fibre exceeds the width of the core, or the axis of the laser beam may not be well aligned with the axis of the core.
A beam-delivery fibre assembly generally terminates at each end in a termination. The termination serves to locate and secure the fibre so that the input and output faces of the fibre are each located at, and remain in, predetermined positions. This is particularly important at the fibre input face in order to preserve the axial and transverse alignment between the incident laser beam and the fibre core. The body and main parts of a termination are generally made of metal for rigidity and to enable any heat in the termination to be conducted away. The fibre is usually secured to the termination by some form of adhesive, cement or glue.
In conventional fibre design, the materials of the fibre buffer and jacket are chosen for properties other than withstanding high levels of optical radiation. The buffer layer for example may be designed to prevent abrasion of and to exclude water vapour from the surface of the cladding layer and to provide a resilient layer for absorbing mechanical impacts. The jacket provides additional mechanical protection and strength. This means that if exposed to high levels of optical radiation, the jacket and buffer layer can both be damaged. Thus laser radiation should not be allowed to impinge directly on the jacket and buffer layers. Accordingly, in order to separate the buffer and the jacket from instant radiation, and in order to facilitate the process of securing the fibre to the termination, it is usual for both the buffer and the jacket to be removed over at least a short length extending from the input face and from the distal face (output face) of the fibre.
The adhesive which is normally used to secure a fibre to a termination can be applied between the cladding and the termination and/or between the jacket and the termination. If it is applied between the cladding and the termination then, since significant laser beam power can be present in the fibre cladding, some of this laser power will transfer from the cladding into the adhesive and some of that power will then transfer from the adhesive into the metal termination. While most commonly used adhesives are tolerant to moderate levels of laser power in that they do not absorb it and therefore do not heat up themselves, the adjoining metal part readily absorbs laser power and therefore gets very hot. This in turn overheats the adhesive and causes it to fail, potentially causing catastrophic damage to the fibre. A similar effect occurs if input radiation misses the core and cladding and impinges directly upon the adhesive.
If adhesive is provided directly between the jacket and the termination, then the adhesive is more remote from the cladding and is therefore less likely to be overheated. However, since the jacket itself is separated from the cladding by the buffer, which is a flexible material, this is not so satisfactory for precisely maintaining the fibre faces at predetermined positions.
Presently, optical beam delivery systems are being used to transmit laser powers of up to 5 kilowatts of continuous wave laser power. It is envisaged that shortly power levels as high as 10 kilowatts mean power, or higher, will be commonplace. If even a small proportion of the beam power is present anywhere other than in the core of the fibre, then there is clearly potential for catastrophic failure, particularly in the terminations.
There have in the past been several attempts at solving the problem of optical radiation, incident on either fibre face, which enters the cladding layer directly, but these are not the subject of the present invention.
At the input end of the fibre, because of gross beam misalignment or an oversize beam for example, the beam may pass over the edge of the cladding and thus impinge on or enter the fibre termination. Optical radiation that passes over the edge of and down the side of the cladding is hereinafter termed xe2x80x9cspillover radiationxe2x80x9d irrespective of the source of the radiation or the reason for the spillover.
Spillover radiation can have a very catastrophic effect on optical fibre assemblies and particularly, fibre termination assemblies.
Spillover radiation can also occur at the output face of an optical fibre. Generally, in order to couple laser radiation from the output face of an optical fibre to a workpiece, the face is imaged onto the workpiece using an optical system which typically comprises two lenses A,B, as shown in FIG. 1. A beam from a laser C is directed by an optical fibre D, via an optical system, to a workpiece E. When the workpiece is in focus, that is in the image plane of the fibre face, then the area of the workpiece on which the laser is incident will also be imaged back on the fibre output face. Since coupling of laser radiation into a workpiece is never perfect, some laser radiation will always be reflected back to the fibre by the workpiece. The degree of back reflection will generally be highest when the laser beam is switched on or first applied to a new area of the workpiece and before the laser beam has had time properly to break down the surface reflectivity of the workpiece. Often, users are advised to tilt the workpiece with respect to the optical system in order to reduce the amount of back reflected laser radiation reaching the output termination.
In addition to back-reflected radiation, there is also a problem arising from broad band optical radiation generated by the interaction of the laser beam with the workpiece. It is well known that the interaction of a beam with a material usually creates a plume of partially ionised gas, resembling a visible flame, which typically extends several millimetres from the workpiece surface. Other radiation is also generated by the interaction. The result is that optical radiation covering a range of frequencies is generated. Much of this generated radiation propagates towards and is intercepted by the fibre imaging optics and so is focused into the region around the output face of the fibre. However, the lateral extent of the focused process radiation is generally larger than the cladding diameter for various reasons, such as the diameter of the plume, chromatic lens aberrations and, when the workpiece is not in focus. These always result in significant spillover radiation which can be very damaging to the fibre termination.
Unlike misalignment of the laser beam at the fibre input face, which can be at least partially corrected by good design and careful alignment, the incidence of process radiation on the output end of the fibre (and therefore of spillover radiation) cannot be eliminated. Accordingly, a significant amount of spillover radiation is inevitable at the output end of a fibre.
Attempts have been made in the past to reduce the damaging effects of spillover radiation. Examples are DE 297 10678; U.S. Pat. No. 5,179,610; EP 0619508 and U.S. Pat. No. 5,291,570. These solutions employed a reflector in the path of spillover radiation. Such reflectors are difficult to make and position, and require coatings which are difficult to manufacture and which are made of materials that have a relatively low optical damage threshold.
The present invention arose in an attempt to avoid the harmful effects of potentially damaging amounts of spillover radiation both at the input and output ends of a high power fibre assembly.
According to the present invention in a first aspect there is provided apparatus for protecting a fibre optic conductor assembly from errant radiant energy, the conductor assembly comprising an optical fibre, said apparatus comprising at least one body mounted adjacent to the fibre so as to provide an alternate path for radiant energy adjacent to the path defined by the optical fibre, the at least one body being configured so that radiant energy within the at least one body is redirected out of said body.
Preferably, radiation absorbing means are provided to absorb the redirected radiation.
According to the present invention in a further aspect there is provided a termination for a fibre optic conductor having a proximal end, comprising at least one body forming an alternate optical path for radiant energy not incident upon said proximal end of the fibre, the at least one body being configured so that radiant energy within the at least one body is re-directed out of said body.
Preferably, the at least one body is shaped to re-direct the radiant energy by virtue of total-internal-reflection. Alternatively radiant energy may be re-directed from said body by refraction or diffraction.
The body is most preferably a tube, such as a capillary tube.
Preferably, means are provided for analysing the re-directed radiation. This can be used to obtain useful information, such as determining whether a beam is in focus on a workpiece, or determining when a materials processing operation should be terminated because harmful levels of spillover radiation are present at a fibre termination.
According to the present invention in a further aspect there is provided a method of protecting a fibre optic conductor assembly, the fibre optic conductor having a proximal end, from errant radiant energy, comprising the steps of providing at least one body adjacent to the fibre conductor wherein said at least one body provides an alternate optical path for radiant energy not incident upon said proximal end; and re-directing radiant energy out of said at least one body.
Preferably, the at least one body into which errant radiant energy passes, has a distal end which is angled so that said radiant energy undergoes total internal reflection and is redirected out of the body and away from the fibre. Alternatively, radiant energy may be re-directed from said body by refraction, or diffraction.